(a) Technical Field
The present invention relates to a polymer electrolyte membrane bonded with an ionic liquid by chemical reaction of the ionic liquid with a novel polymer chain terminal, and a fuel cell using the polymer electrolyte membrane.
(b) Background Art
Hydrogen conductive polymers have been widely studied for use in a polymer exchange fuel cell (PEFC), which supplies an environmentally friendly (e.g., eco-friendly) source of energy. One of the main problems for the development of hydrogen-conductive polymer technology is the ability to produce a hydrogen-conductive polymer that has long-term stability and durability. Unfortunately, the stability and durability of such polymers is negatively impacted by a variety of factors, including: carbon monoxide pollution occurring in a platinum catalyst, complexity in heat and water control systems, moisture maintenance in a polymer electrolyte, and improvement of reaction speed in an electrode.
The simplest approach to solve the foregoing problems is to improve the operating temperature of the PEFC. It is well known that carbon monoxide pollution in an electrode decreases to the ignorable level as operating temperature increases. However, such an increased operating temperature is higher than, or equal to, the boiling point of water, which requires a low humidification condition. Therefore, there is a need to develop a system in which a new hydrogen conductor having a high boiling point and non-volatile property is introduced in place of water.
At present, Nation® (DuPont) is mostly used as a polymer membrane of a polymer electrolyte fuel cell that operates at temperatures in the range of 60-80° C. Nation® is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, which is a synthetic polymer with ionic properties (e.g., ionomers). Similar polymer membranes may include, for example, Flemion® (Asahi Glass), Aciplex® (Asahi Kasei), etc., however, such polymer membranes are not commercially viable for use in a fuel cell-based application because of their prohibitively expensive price.
In order to reduce cost, a hydrocarbon-based electrolyte that introduces a sulfonic acid group or phosphonic acid group to a polymer having superior thermal stability and mechanical strength has been developed and actively studied. For example, aromatic polyether, which is a representative, attractively priced engineering plastic, is a polymer in which a phenylene ring is Connected to an oxygen atom, is. The aromatic polyester in its humidified state shows high hygroscopic property and high hydrogen conductivity. Unfortunately, at high temperature its performance significantly degrades due to the evaporation of water.
As to a conventional polymer electrolyte fuel cell, one suggested conventional art solution proposes a fuel cell consisting of an electrolyte membrane including an ionic conductive film between a nitrogen-containing compound, which contains histamine, and an ionic conductive polymer. Another suggested conventional art solution proposes a fuel cell asymmetric membrane that is a complex of polyarylene having a sulfonic acid group and a nitrogen-containing compound (histamine). However, the foregoing proposed solutions have the disadvantage of extremely low mechanical strength and/or leakage of the nitrogen-containing compounds from the polymer electrolyte membrane during the course of long-term use of the soaked nitrogen-containing compound.
Recently, as part of an effort to develop a polymer electrolyte substance operable in an anhydrous/high temperature environment, the present inventors have measured conductivities of various types of ionic liquids soaked in the PSS-PMB polymer electrolyte. By using alkylimidazole salt having high thermal stability, it has been found that various nano structures are formed according to a molecular weight of a polymer and a type and a relative content of a soaked ionic liquid. Moreover, the conductivity of the resulting nano structures vary largely depending upon the type of the nano structure.
By discovering such a correlation, with different molecular weights and degrees of sulfonation of block copolymers and different types and soaking of ionic liquids, conductivities were measured and correlations among them were also investigated, resulting in a high conductivity of 0.045 S/cm which is the highest conductivity level known to occur at a high temperature of 165° C. For example, such a conductivity level is three times greater than the conductivity of Nafion®, which has a maximum conductivity of 0.014 S/cm at 165° C. Disadvantageously, this system experiences significant degradation of mechanical strength because the soaked ionic liquid absorbs moisture in a humidified environment, which causes the soaked ionic liquid to leak out of the electrolyte membrane.